Irrigation control system

ABSTRACT

An irrigation control system for land comprising: (a) at least one meter to measure one or more wheather conditions in a first area; (b) at least one monitor to (i) examine rainfall data derived from a radar scanning at least the first area according to predetermined criteria and (ii) to extract data which is representative of the scanned rainfall in a sub-area of the first area; (c) a store to store the extracted data; and (d) a controller connected directly or indirectly to the meter and the monitor and to the store, to calculate a moisture content value for the sub-area and a predetermined moisture content value for the sub-area, and to regulate the irrigation in a sub-area.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a U.S. national application of internationalapplication Ser. No. PCT/AU99/00175 filed Mar. 19, 1999, which claimspriority to Australian Provisional Patent Application No. PP2475 filedMar. 20, 1998.

FIELD OF THE INVENTION

The invention relates to an irrigation control system for apredetermined area. In particular, scheduling and management of urbanirrigation of parks, gardens and sports facilities of a large city.

BACKGROUND OF THE INVENTION

Controllers to start and stop irrigation cycles without humanintervention are well known. These controllers send an electric current(usually 24 volt alternating current in horticultural or agriculturaluse) to a remote solenoid valve, causing the valve to open. Valveclosure is usually effected by discontinuing the supply of electriccurrent to the solenoid of the valve whereupon the valve is caused toclose.

Most of these types of controllers are able to handle a number ofvalves, opening and closing them in a programmed succession forprogrammed times on programmed days of the week. This series ofsequential valve opening and closing on specified days is generallyreferred to as “a program” or “an irrigation program”. Many of the knowncontrollers are capable of storing and executing more than oneirrigation program, which adds a degree of flexibility to what thecontroller may accomplish.

Basically these prior controllers fall into one of three categories asfollows:

1. Relatively inexpensive controllers which are capable of executing anirrigation program. These controllers are not capable of changing theset irrigation program in any way to take account of differing waterneeds of plants occasioned by variations in meteorological conditions.

Controllers of his type constitute well over 90 percent of allirrigation controllers currently in use in Australia. Such controllerswill, if the irrigation program is not regularly modified inevitablywaste considerable quantities of water, since it will be programmed tosupply sufficient water to serve the needs of the plant being irrigatedduring periods when plant demand for water is high. Thus when the sameapplication of irrigation water continues during periods of low plantwater requirement. wastage occurs.

The potential to save water by in effect harvesting rainfall bydiscontinuing irrigations until that rainfall finds its way into theroot-zone and is transpired by the plants, is lost unless the controllercan be manually de-activated. When managing large numbers of suchcontrollers, particularly over a wide area, it is generally not possibleto manually de-activate them and re-activate them when irrigation shouldcommence.

Additionally, such controllers are incapable of responding to occurrenceof rain periods unless coupled to some specialist sensor designed forthe purpose. Whilst such sensors are known they tend to be eitherexpensive (and consequently little used) or unreliable (and again littleused).

2. More expensive controllers which can alter the frequency and amountof irrigation, either up or down, as time passes in an effort to matchapplications to plant requirements. Such devices usually impute likelyplant requirements by use of meteorological averages developed fromexamination of many years of meteorological records relating to thegeographical area under consideration. This type of controller is animprovement upon the first described type of controller, but is stillarbitrary and inflexible as it relies on averages that must inevitablywaste water when the predicted conditions do not occur. Additionally,there can be no improvement in harvesting rainfall.

3. Expensive controllers which either accept direct input from automaticweather stations, or accept meteorological information directly orindirectly from a remote weather station or climatic recording facility.These controllers use such information to modify a basic program so thatirrigation water applications are substantially in accord with actualplant requirements. These controllers may also be activated to apply apredetermined irrigation cycle when instructed to do so by a remotesoftware program which accepts meteorological input and maintains awater budget for the area. However, such controllers do not utiliselocalised rainfall measurement and consequently irrigation managementdepends upon rainfall information indicative of a wider area than theirrigation area. Water wastage can result. Further, these controllersmust be part of a very wide network which means that over a wide areavery considerable telephony or radio costs are necessarily involved.

Another approach is described in our pending patent application no.PCT/AU97100056 the content of which is incorporated herein. In thatapplication, the control system is based upon a method of irrigatingland which includes the steps of:

-   -   (a) measuring one or more weather conditions in a first area;    -   (b) measuring rainfall in a sub-area of the first area;    -   (c) monitoring the measurements;    -   (d) calculating a moisture content value for the sub-area from        the measurements and a predetermined moisture loss for the        sub-area; and    -   (e) regulating the irrigation of the sub-area.

Key to this approach is the combined use of one or more weatherconditions in the first area and the rainfall in the sub area. Inimplementing certain forms of that invention, it has become apparentthat where large numbers of sub-areas (such as parks, gardens and sportsfacilities) need to be managed by the system, special practical economicdifficulties may arise.

If individual actual measurement is needed of a large number ofsub-areas, it would be necessary to place at least one weather stationincluding a tipping bucket (or other) type of pluviometer, in anappropriate position in each sub-area. This could be as often as 500meters apart. However, weather stations are expensive and a very commoncharacteristic of rainfall is that it can be extremely variable inamount and distribution even over a small area. For example, well over1000 stations would be needed in even a small city to establish anetwork. Thus to produce reliable data using such pulviometers is anexpensive undertaking.

Accordingly, improved and more economic resolution of rainfallinformation over a wide area is important in the management ofindividual sub-areas. In addressing this issue it has been found thatone source of potentially useful data for this purpose is readilyavailable.

In this respect, at present meteorological weather radars are commonlyinstalled to cover the area of major cities. The output of these radarsis a stream of data organised in the following fashion:

-   -   Radially—each degree of rotation from 000 through to 360 is        reported separately.    -   Longitudinally—for each degree of rotation, data is presented as        a series of rain intensity figures, typically for each kilometre        along each of the radii. For example 095/35/9 may mean that rain        intensity of “9” is falling 35 kilometres from the radar        transmitter on a bearing of 095 degrees from the transmitter.

This data is analysed by a high-speed computer to produce the familiarradar screen views commonly seen on television weather reports.

OBJECT OF THE INVENTION

It is an objective of the present invention to provide an irrigationcontrol system which ameliorates the disadvantages referred to aboveespecially where there is a large number of sub-areas which need to becontrolled.

SUMMARY OF THE INVENTION

According to one form of the invention, an irrigation control system forland is provided which comprises:

-   -   (a) at least one meter to measure one or more weather conditions        in a first area;    -   (b) at least one monitor to (i) examine rainfall data derived        from a radar scanning at least the first area according to        predetermined criteria and (ii) to extract data which is        representative of the scanned rainfall in a sub-area of the        first area;    -   (c) a store to store the extracted data; and    -   (d) a controller connected directly or indirectly to the meter        and monitor and to the store, to calculate a moisture content        value for the sub-area and a predetermined moisture content        value for the sub-area, and to regulate the irrigation in a        sub-area.

Preferably, regulation of irrigation in the sub-area is either byinitiating or preventing irrigation of the sub-area depending uponwhether the moisture content value is less than or more than apredetermined moisture content value for the sub-area.

Typically, there will be one monitor.

Preferably the weather conditions measured include solar radiation.

Preferably, the monitor is integrated with the controller.

Preferably, the controller is a computer.

According to a preferred form of the invention, the irrigation controlsystem further comprises a local switch in the sub-area to initiate orprevent irrigation in response to signals from the controller.

According to another preferred form of the invention, the local switchin the sub-area activates or de-activates a local controller forinitiating or preventing the irrigation, in response to signals from thecontroller.

According to another preferred form of the invention, the irrigationcontrol system further comprises an interrupter to interrupt irrigationin the sub-area. Preferably, this interrupts irrigation in the sub-areain response to rainfall in the sub-area. Typically, the interruptionoccurs for a period of time determined by the controller.

In another independent aspect of the invention, a method of irrigatingland is provided which comprises the steps of:

-   -   (a) measuring one or more weather conditions in a first area;    -   (b) examining rainfall data derived from a radar scanning at        least the first area according to predetermined criteria and        extracting data which is representative of the scanned data in a        sub-area of the first area;    -   (c) storing the extracted data;    -   (d) calculating a moisture content value for the sub-area and a        predetermined moisture content value for the sub-area; and    -   (e) regulating the irrigation of the sub-area.

Preferably the regulation of the irrigation of the sub-area is either byinitiating or preventing irrigation of the sub-area depending uponwhether the moisture content value is less than or more than apredetermined moisture content value for the sub-area.

Preferably, the measurement in step (a) is carried out in the samesub-area as that in which the measurement is carried out in step (b).

Preferably the method comprises a further step of: (f) sensing forrainfall in the sub-area during irrigation and interrupting irrigationin response to rainfall in the sub-area.

Using the system and method described above, it is possible to moreaccurately manage the irrigation of an area and minimise over irrigationand hence wastage of water.

DESCRIPTION OF A PRACTICAL EMBODIMENT

The invention will now be further explained and illustrated by referenceto the following practical embodiment concerning steps (b) and (c) ofthe control system when dealing with the radar data.

As stated above, at present meteorological weather radars are commonlyinstalled to cover the area of major cities. In the current example thisarea would be seen as the first area. The output of these radars is astream of data organised in the following fashion:

-   -   Radially—each degree of rotation from 000 through to 360 is        reported separately.    -   Longitudinally—for each degree of rotation, data is presented as        a series of rain intensity figures, typically for each kilometer        along each radii. For example 095/35/9 may mean that rain        intensity of “9” is falling 35 kilometers from the radar        transmitter on a bearing of 095 degrees from the transmitter.

This radar data may be processed as follows according to one aspect ofthe invention.

1. Firstly a “plane geometry” program is created and initialised. Thissoftware program, on being programmed with the longitude and latitude ofthe radar transmitter, is able to express any longitude and latitudepair in the scanned range of the transmitter, as a radial and longitudeaddress in terms of the data stream emanating from the radartransmitter. This results in the production of weather radar dataparticular to any site (sub-area) within the coverage area (first area)of the radar. This in turn means such particular data may be identified,extracted and stored along with a time stamp by a computer.

2. The size and pattern of the network of rainfall assessments in thesub-areas to be managed is decided upon and programmed into the planegeometry software program to create the sites of a virtual rain gauge.

3. The latitude and longitude of each virtual rain gauge can now beestablished and accurate radar addresses for each site can be computed.

4. The radar address of each site can then be stored and aninterrogating computer programmed to extract and store the data for eachaddress so identified every time it appears in the data stream comingfrom the radar transmitter. If the radar beam sweeps through 360 degreesevery 10 seconds, then one piece of data will be stored for each virtualrain gauge site each 10 seconds.

5. This rainfall intensity data is examined for the existence ofsignificant rain by integrating this information with time.

Consequently, by use of this technique, rainfall data for any array ofpoints defined within the area of coverage of a weather radar can beinexpensively collected without the necessity for expensive actual rainmeasurement equipment. This makes possible a variety of irrigation/watermanagement techniques which would otherwise be impossible due to lack ofresolution of rainfall data.

The invention will now be further explained and illustrated by referenceto the accompanying drawing in which:

FIG. 1 is a schematic diagram of one form of the invention.

In overview, the system depicted includes a controller which is a maincomputer 1 which communicates directly or indirectly with at least oneradar 11. The main computer 1 examines rainfall data derived from theradar 11 which scans at least the first area 2 according topredetermined criteria. The main computer 1 also examines and extractsdata from the radar 11 which is representative of the scanned rainfallin a sub-area 3 of the first area 2. There is at least one weatherstation 12 in the first area 2. The weather station 12 can measureweather conditions such as radiation and temperature. Main computer 1also communicates with the weather station 12 and can calculate themoisture content value for a particular sub-area 3.

Main computer 1 also communicates with one or more switches 4 typicallyby a paging network 15. Each of these switches is associated with apreexisting irrigation controller for a particular tract of land. Eachswitch 4 is controlled by the main computer 1 and closing or openingswitch 4 permits or prevents the pre-existing irrigation controller 5from irrigating the tract of land according to its own programmed cycle.The pre-existing irrigation controller 5 may have field wires 16 whichcan be used to activate irrigation in each sub area 3.

It will be understood by those skilled in the art that a system forirrigation can be set up in a variety of ways. Two methods areparticularly described as ‘fail wet’ or ‘fail dry’. In a fail wetsystem, the resting state of the system is such that irrigation ispossible. In the preferred form of a fail wet system, at about 7.00 pmall switches are activated and remain in this state until the preferredirrigation period is finished. In this state irrigation is prevented. Itis necessary for the controller 1 to change the state of switches 4 forthose sub-areas 3 which require irrigation. The circuits for thesesub-areas 3 then enable. Once irrigation has concluded the switches 4will return to the ‘off’ state. When the irrigation period as a whole isfinished all circuits will return to their resting state-ie. irrigationpossible. Water, however, will generally not be supplied during thisperiod, except as may be required by on-site supervisors for particularpurposes.

A fail dry system is opposite to a fail wet system.

It will be understood that both fail wet and fail dry systems can beutilised with this invention. However, fail wet systems have theadvantage of more easily permitting the testing of the system during theperiod when the switches are in their resting state. As a general rule,irrigation will preferably occur during the evening or night as it iscoolest then and less of the water provided by way of irrigation islost. If this is the case the switches 4 will be in their resting stateduring the day when it is most likely workmen would be testing them.

Where users require external access to the system, to review and/oralter the irrigation control system settings for a particular site, adial up facility is provided. External users 10 will access a network ofcomputers 6 via a call sequencer 7. The user 10 enters by securityidentification and identification of the particular site. The user 10 isthen given access to the weather information and the settings for thatparticular site which have been down loaded to the network computer 6from the main computer 1. The user 10 may change the settings and thisinformation is then transmitted to the main computer 1.

A typical implementation procedure for the irrigation control system asdepicted is as follows.

The area 2 in which the irrigation is to be controlled is defined. Inmost cases this will be the greater metropolitan area and environs of alarge city, or the general area and environs of a provincial city, orthe area covered by a town.

Once the overall area is defined, it is further divided into sub-areas3. A sub-area 3 is defined by a common or similar microclimate.Sub-areas may also be defined by areas which provide differentfacilities ie. different sports facilities or parks. This division isnecessarily subjective and will usually contain inaccuracies, however,this does not markedly affect the operation of the system and does notinterfere with the system achieving efficient irrigation managementoutcomes. It is possible to ‘fine tune’ the system if necessary.

Typically, a large metropolitan area and environs of a city, eg onemillion people, may contain 10-15 sub-areas 3. These sub-areas 3 will beidentified by a number.

To define sub-areas 3, a number of empirical factors are used including:

-   -   General orientation (North, South etc.)    -   Landform (plain, valley area, slope)    -   Overall land use    -   Density of buildings, etc.

Once the overall area has been defined and then further sub-divided into(10-15) sub areas 3, the following external support network is put inplace. The radar 11 scans the entire first area 2 according topredetermined criteria. This is done via radar ‘communication’ system14. The main computer 1 examines the scanned rainfall data derived fromthe radar 11. The scanned rainfall data may be stored by the radar 11 orby the main computer 1. Alternatively it may be stored elsewhere. Themain computer 1 also examines and extracts data which is representativeof scanned rainfall in a sub-area 3 of the first area 2. The radar 11 isconnected to main computer 1 by communication system 13. Each weatherstation 12 is also connected to main computer 1 by communication system8 (usually telephone or radio or a combination of both). As indicatedabove, at least one weather station 12 may be installed in the overallfirst area 2 and not specifically in each sub-area 3. In anotheralternative (not shown) it is possible to install one weather station 12within each sub-area 3. In another alternative (not shown) there may bea combination of those alternatives.

Each site to be irrigated is surveyed with a view to accuratelyestablishing the following:

-   -   Area (sq.m) to be irrigated.    -   Root Zone Depth (RZD). This is a sensible site range.    -   Precipitation Rate (or rates) of the irrigation system.    -   Soil texture (or textures) within the root zone.

With the data from the above, calculations can now be done to establishthe following:

-   -   Total Available Water (TAW (mm)=RZD(cm)×SMHC where SMHC (Soil        moisture holding capacity is typically 0.75 mm/cm for sand; 1.00        mm/cm for sandy loam; 1.40 mm/cm for loam; 1.60 mm/cm for clay        loam and 1.80 mm/cm for clay).    -   Refill Point (RFP(mm)=TAW(mm)×f where f is a factor, 0.5 has        been found satisfactory for many soils. The RFP is the        predetermined moisture content value. It will be understood by        those skilled in the art that the factor f varies depending on a        variety of circumstances. However, f will generally fall within        the range of 0.4 to 0.6.    -   Optimum Irrigation Event        (OIE(mins)=((TAW(mm)−RFP(mm))/PR(mm/hr)×60 where PR is        Precipitation rate (mm/hr).

It will be understood by those skilled in the art that further factorsmay be taken into account to account for local variance.

Each individual site is now registered on the main computer 1 with itsbasic factors indicated above (TAW, RFP, OIE, PR) and its identificationnumber which tells the system within which sub-area 3 it lies.

The switch 4 is now connected to the pre-existing irrigation controller5 at each site. Switch 4 is connected across the common wire (or wires)or active bus 9 of pre-existing irrigation controller 5. It will howeverbe understood that this can be done in a variety of ways.

Now a program is entered into the preexisting irrigation controller 5which calls for the calculated Optimum Irrigation Event to be applied toeach site the first 24 hours. Typically this will take place at night.

In this arrangement, the pre-existing irrigation controller 5 is allowedto operate on the first night so that the site is “zeroed” by having itsroot zone filled with all the moisture it is able to store.

From then on the programming in the main computer 1 maintains the soilmoisture budget for each of the sites registered onto it. It does thisby communicating with the weather station 12 and the radar 11 andestablishing how much water will have been lost by each sub-area 3, i.e.by transpiration of the plants. Loss of water is calculated byconventional methods. This amount is deducted from the soil moisturebudget (TAW initially and then the soil moisture budget as calculated atthat time) of each site, with additions to the soil moisture budget foreach site being made where rain falls in a particular sub-area 3. Theresult of these calculations is the moisture content value.

When the soil moisture budget being maintained by the system for eachsite indicates that the soil moisture content has fallen to the RefillPoint (predetermined moisture content value) for a particular site, thatsite is placed in an Action List for that day. Sites on the Action Listfor any particular day are activated by the system in early evening.This is done by the main computer 1 sending a data string through thenumeric paging network of an appropriate telecommunications providersuch that the switch 4 changes state as required for a programmed period(typically but not necessarily 20 hours) thus allowing irrigation tooccur during that 20 hour period under the influence of pre-existingirrigation controller 5.

After typically 20 hours the switch 4 changes state, which terminatesirrigation. Irrigation will not occur again until the data string isonce more received from main computer 1 causing the switch 4 to changestate and pre-existing irrigation controller 5 to activate irrigation.The soil moisture budget of sites on the Action List is altered toreflect the receipt of the Optimum Irrigation Event during the followingday, provided that the irrigation was not interrupted by rain.

If this is a fail wet system, and irrigation is required, main computer1 is required to energise the requisite switch 4. This will occur duringthe preferred irrigation time, ie. often at night. Irrigation during thenon-preferred irrigation period in such a fail wet system, merelyrequires the pre-existing irrigation controller 5 to ensure provision ofwater to the relevant area, as the switches 4 are in a state whichallows irrigation to occur.

Should rain fall, it will be detected by the radar 11. If it is outsidethe programmed operating window of pre-existing irrigation controller 5,it will be passively recorded and up loaded as data each day by the maincomputer 1. It will then be added to the soil moisture budget of sitesin the main computer 1 which recorded the rainfall. Rainfall willtherefore delay irrigation until it has been transpired or harvested bythe plants or evaporated.

If the rainfall is within the operating window of the pre-existingirrigation controller 5 (that is when irrigation is likely to beoccurring) the data accumulating logger connected to (or part of) theradar 11 will contact the main computer 1 and advise that rain isfalling in that sub-area 3 or at a particular site. In this case allsites from that sub-area 3 on that evening's action list will be sent adata string causing the switch 4 to change state, thus bringingirrigation within that sub-area 3 to an immediate halt. In the case ofan individual site within a sub-area 3, the switch at the site willchange state to prevent irrigation at that site. It will therefore beclear that in some situations it will be possible to independentlycontrol sites within a sub-area if desired. This may in somecircumstances be done by a broadcast call which will be acted upon byall switches 4 in the designated sub-area 3. The main computer 1 willthen track the rainfall event and add it to the soil moisture budget ofthe relevant sites.

The switch 4 operates in the common wire 9 of pre-existing irrigationcontroller 5. This associated pre-existing irrigation controller 5 canbe an inexpensive controller which may have been installed on theirrigation system to be managed prior to adopting the irrigation controlsystem of the invention.

The switch 4 typically consists of, but is not limited to, thefollowing:

-   -   a paging system receiver    -   a microprocessor    -   a memory area    -   a clock    -   one or more switching relays.

Switch 4 is capable of receiving a detailed program containing switchinginstructions for the operation of one or more of the relays. It iscapable of receiving a particular string which is intended for it alone,or depending on the structure or content of the transmitted data stream,it can also respond to a broadcast type call intended to simultaneouslygive rise to a specific action or group of actions within an entiregroup of switches 4.

The relay of the switch 4 may be either of the normally open or normallyclosed type depending upon the circumstances.

Further switch 4 is capable of receiving, processing and storing datastrings including (but not limited to) the following types ofinformation, which would normally be transmitted (but not necessarily)in the following order:

-   -   1. General call or broadcast recognition characters    -   2. Sub-area identification number.    -   3. Specific unit recognition or capture code (characters).    -   4. Specified task designation characters (normally used to        designate tasks the subject of a broadcast call).    -   5. Program definition characters of the general type (but not        limited to)—relay one close/open at (time) for duration        (minutes); on (date);—relay a, close/open at (time); for        duration (minutes); on (date).    -   6. Test time.    -   7. Lock/unlock code (prevent all irrigation operations until        receipt of particular unlock code).

Also included in switch 4 may be an accessible momentary switch which,if pressed or otherwise operated, will allow irrigation operations inthe absence of system authorisation for a programmable “Test” time. Inother words the switch 4 will restore the integrity of the circuits.This is to allow the associated pre-existing irrigation controller 5 andits in-field irrigation system to be tested.

The word ‘comprising’ and forms of the word ‘comprising’ as used in thisdescription and in the claims does not limit the invention claimed toexclude any variants or additions which are obvious to the personskilled in the art and which do not have a material effect upon theinvention.

Modifications and improvements to the invention will be readily apparentto those skilled in the art. Such modifications and improvements areintended to be within the scope of this invention.

1. An irrigation control system for land comprising: (a) at least onemonitor to (i) examine rainfall data derived from a radar scanning atleast a first area according to predetermined criteria and (ii) toextract weather data which is representative of the scanned rainfall ina sub-area of the first area; (b) storage device to store the extracteddata; and (c) a controller connected directly or indirectly to themonitor and to the storage device to calculate a moisture content valuefor the sub-area based on said rainfall data, and to regulate theirrigation in a sub-area in accordance with said moisture content. 2.The irrigation control system of claim 1 wherein regulation ofirrigation in the sub-area is either by initiating or preventingirrigation of the sub-area depending upon whether the moisture contentvalue is less than or more than the predetermined moisture content valuefor the sub-area.
 3. The irrigation system of claim 1 wherein there isone monitor.
 4. The irrigation system of claim 1 wherein the weatherconditions measured include solar radiation.
 5. The irrigation system ofclaim 1 wherein the monitor is integrated with the controller.
 6. Theirrigation system of claim 1 wherein the controller is a computer. 7.The irrigation system of claim 1 wherein the irrigation control systemfurther comprises a local switch in the sub-area to initiate or preventirrigation in response to signals from the controller.
 8. The irrigationsystem of claim 7 wherein the local switch in the sub-area activates orde-activates a local controller for initiating or preventing theirrigation, in response to signals from the controller.
 9. Theirrigation system of claim 7 wherein the irrigation control systemfurther comprises an interrupter to interrupt irrigation in thesub-area.
 10. The irrigation system of claim 9 wherein the interrupterinterrupts irrigation in the sub-area in response to rainfall in thesub-area.
 11. The irrigation system of claim 9 wherein the interruptionoccurs for a period of time determined by the controller.
 12. A methodof irrigating land is provided comprising the steps of: (a) measuringone or more weather conditions in a first area; (b) examining rainfalldata derived from a radar scanning at least the first area according topredetermined criteria and extracting weather data which isrepresentative of the scanned rainfall in a sub-area of the first area;(c) storing the extracted data; (d) calculating a moisture content valuefor the sub-area based on said rainfall and a predetermined moisturecontent value for the sub-area; and (e) regulating the irrigation of thesub-area.
 13. The method of claim 12 wherein the regulation of theirrigation of the sub-area is either by initiating or preventingirrigation of the sub-area depending upon whether the moisture contentvalue is less than or more than the predetermined moisture content valuefor the sub-area.
 14. The method of claim 12 wherein the measurement instep (a) is carried out in the same sub-area as that in which themeasurement is carried out in step (b).
 15. The method of claim 12wherein the method comprises a further step of: (f) sensing for rainfallin the sub-area during irrigation and interrupting irrigation inresponse to rainfall in the sub-area for a period of time controlled bythe duration and amount of rainfall.
 16. The irrigation system of claim2 further comprising at least one meter to measure one or more weatherconditions in said first area wherein the weather conditions measuredinclude solar radiation.
 17. The irrigation system of claim 10 whereinthe interruption occurs for a period of time determined by thecontroller.